The present invention is directed to a method of making aluminum oxide articles, and more particularly to a method of reducing magnesium loss that occurs during sintering of an aluminum oxide discharge vessel for a high-intensity-discharge (HID) lamp.
HID lamps such as high-pressure-sodium (HPS) lamps and ceramic metal halide lamps generally employ a ceramic discharge vessel made of a translucent polycrystalline aluminum oxide (PCA) for arc generation and containment. These discharge vessels (e.g., arc tubes in HPS lamps) are fabricated from a relatively pure aluminum oxide (alumina) powder and are typically formed by sintering at temperatures over 1800° C. in a hydrogen atmosphere. These conditions lead to the high total light transmission in the sintered discharge vessels that is required for lighting applications.
High sintering temperatures will cause exaggerated grain growth and reduced material strength unless controlled. A method to control grain growth in aluminum oxide has been to add small amounts of magnesium oxide to the aluminum oxide. Typically, the amount of magnesium oxide ranges from 0.02 to 0.1 weight percent (wt %) of the aluminum oxide. Some of the added magnesium oxide leaves the ceramic during sintering through vaporization in the hydrogen sintering atmosphere.
Magnesium may also be lost during the operation of the lamp and has been linked to blackening of the outer jacket glass of high pressure sodium lamps. It is therefore desirable to use as little magnesium oxide as possible for maintaining control of grain growth. Experiments have shown that the use of 0.02 wt % magnesium oxide in the aluminum oxide is about the lowest level that can be processed successfully on a production scale without risking exaggerated grain growth. However, at this level localized exaggerated grain growth may occur unless another source of magnesium oxide is added to provide a greater partial pressure of magnesium in the furnace atmosphere. The additional magnesium reduces the loss of magnesium from the discharge vessels into the furnace atmosphere.
Prior attempts to reduce magnesium loss involved adding magnesium oxide powder to the sintering furnace, but this proved difficult to control. Some regions in the sintering furnace had excessive magnesium levels and other regions had too little. The powdered magnesium oxide would also adhere to molybdenum parts of the sintering furnace and create a rough surface contacting the discharge vessels.
A further method was developed to produce a granular powder enriched in magnesium oxide that provided better control of the magnesium release during sintering. The method added magnesium oxide to aluminum oxide powder at a level of 10 wt %. The process included mixing the powders in a concentrated nitric acid solution, followed by a slow drying and a prefiring process. The resulting material was then crushed into a powder that could be placed in several locations in the sintering furnace. This method was generally satisfactory, but required the handling of concentrated acid, a long drying time, and still left a powder buildup on the molybdenum sintering fixtures.
Accordingly, it is desirable to have an improved process for adding magnesium to the sintering furnace atmosphere to reduce the loss of magnesium that occurs during sintering of aluminum oxide articles.